Electromagnetic actuators are conventionally used as the driving sources for acoustic devices such as speakers. An electromagnetic actuator is constituted by a permanent magnet and a voice coil, and generates a vibration by the function of the magnetic circuit of a stator using the magnet. Further, electromagnetic speakers generate sound by vibrating a low-rigidity diaphragm such as an organic film fixed on the vibrating part of the electromagnetic actuator.
In recent years, as the demands for mobile telephone devices and notebook-type personal computers have increased, the demands for small-size and low-power consumption actuators increase. However, since electromagnetic actuators need a large amount of current flowing into the voice coil during operation, it is difficult to reduce the power consumption. Moreover, electromagnetic actuators are not structurally suitable for miniaturization. In addition, an electromagnetic actuator must be covered with an electromagnetic shield when used in an electronic apparatus in order to prevent harmful effects caused by a leakage magnetic flux from the voice coil, and it is not suitable for use in small-size electronic apparatuses such as mobile telephones from this reason too. Finally, the voice coil becomes thinner as the size is reduced, and as a result, there is a probability that the voice coil is burn-damaged due to an increase in the resistance value of the wire.
In order to solve the problems above, a piezoelectric actuator having the characteristics of being small, light, power-saving, and no leakage of magnetic flux and having a piezoelectric element such as a piezoelectric ceramic as the driving source has been developed as a thin vibration part that replaces the electromagnetic actuator. The piezoelectric actuator generates a mechanical vibration through the motion of the piezoelectric element, and for instance, it has a configuration in which the piezoelectric ceramic (or simply “piezoelectric element”) is joined to a base.
The basic structure and operation of a piezoelectric actuator will be described with reference to FIGS. 58 and 59. FIG. 58 is a schematically exploded perspective view showing the configuration of a piezoelectric actuator in the background technology, and FIG. 59 is a schematic diagram showing how the piezoelectric actuator in FIG. 58 vibrates.
As shown in FIG. 58, the piezoelectric actuator 311 has a piezoelectric element 312 constituted by a piezoelectric ceramic, a base 313, onto which the piezoelectric element 312 is fixed, and a ring-shaped support member 314 that supports the outer peripheral section of the base 313. When an AC voltage is applied to the piezoelectric element 312, the piezoelectric element 312 starts to expand and contract. As shown in FIG. 59, the base 313 deforms into a convex mode (indicated by the solid line) or a concave mode (indicated by the broken line) according to the expansion and contraction movements. As described, the piezoelectric actuator 311 vibrates vertically as shown in the drawing with the joint section between the base 313 and the support member 314 as a fixed end (node) and the center of the base 313 as the anti-node of the vibration.
A piezoelectric actuator can be made thinner, however, in one aspect, an acoustic performance as an acoustic device is inferior to an electromagnetic actuator. This is because the amplitude of a piezoelectric actuator is low at frequencies other than the resonant frequency, although a large amplitude can be obtained from a piezoelectric element at frequencies near the resonant frequency due to its high mechanical Q value and high rigidity, compared to an electromagnetic actuator. Since a low amplitude of an actuator translates into a low sound pressure, this means that a sufficient sound pressure cannot be obtained in a wide frequency band required for music playback. Further, in order to solve a problem that a piezoelectric actuator has a low sound pressure in a frequency band not higher than the fundamental resonant frequency, the sound radiation surface may be enlarged, however, this solution is not preferable when the actuator is built into a mobile electronic apparatus. Meanwhile, for instance, Patent Documents 1 to 7 disclose means for increasing the vibration amplitude of a piezoelectric actuator.
A piezoelectric actuator described in Patent Document 1 comprises a first bimorph and a second bimorph formed shorter than the distance between two support ends of the first bimorph. An end of the second bimorph is fixed on position [sic. one] surface of the first bimorph in an insulated state so that the second bimorph vibrates in the same direction as the thickness of the first bimorph.
In a piezoelectric actuator described in Patent Document 2, the peripheral part of a vibrating body constituted by a piezoelectric body and an elastic body is supported and fixed by a spring structure.
In a piezoelectric actuator described in Patent Document 3, a piezoelectric body is supported by a support member via an elastic member, and a slit is formed inside the elastic portion, between the elastic member and the support member, or between the elastic member and the piezoelectric body.
In a piezoelectric acoustic device described in Patent Document 4, the peripheral part of a piezoelectric vibrator is fixed to the inner circumference of a ring-shaped support member, the outer circumference of the support member is fixed to the peripheral wall of a case, and the support member is constituted by a planar member and has a curved section curving in the thickness direction between the inner circumference and the outer circumference.
An electroacoustic transducer for a parametric speaker described in Patent Document 5 comprises a piezoelectric actuator, a diaphragm, bonded underneath the piezoelectric actuator, that vibrates so as to generate ultrasonic waves along with the flexural vibration of the piezoelectric actuator, and a resonator that resonates the ultrasonic waves generated with the vibration of the piezoelectric actuator, and the resonator has a resonance chamber and a sound output hole that goes through the resonance chamber.
A spherical piezoelectric speaker described in Patent Document 6 comprises a spherical shell type of a piezoelectric ceramic body, which is hollow and is polarized in the thickness direction, an external electrode disposed on the outer surface of the piezoelectric ceramic body, and an internal electrode disposed on the inner surface of the piezoelectric ceramic body, and sound is generated by supplying a drive signal across the external electrode and the internal electrode so that the piezoelectric ceramic body oscillates.
In a piezoelectric sounding body described in Patent Document 7, one end of a piezoelectric vibrating body is supported by a support body on a sound panel, and a vibration transmitting material is fixed between the other free end and the sound panel.    [Patent Document 1] Japanese Patent Kokai Publication No. JP-A-61-168971    [Patent Document 2] Japanese Patent Kokai Publication No. JP-P2000-140759A    [Patent Document 3] Japanese Patent Kokai Publication No. JP-P2001-17917A    [Patent Document 4] Japanese Patent Kokai Publication No. JP-P2001-339791A    [Patent Document 5] Japanese Patent Kokai Publication No. JP-P2004-312395A    [Patent Document 6] Japanese Patent Kokai Publication No. JP-A-9-163498    [Patent Document 7] Japanese Patent Kokai Publication No. JP-P2007-96423A